Abstract
Revealing the interactions between alien species and native communities is central to understanding the ecological consequences of range expansion. Much has been learned through study of the communities developing around invading herbivorous insects. Much less, however, is known about the significance of such aliens for native vertebrate predators for which invaders may represent a novel food source. We quantified spatial patterns in native bird predation of invading gall-inducing Andricus wasps associated with introduced Turkey oak (Quercus cerris) at eight sites across the UK. These gallwasps are available at high density before the emergence of caterpillars that are the principle spring food of native insectivorous birds. Native birds showed positive spatial density dependence in gall attack rates at two sites in southern England, foraging most extensively on trees with highest gall densities. In a subsequent study at one of these sites, positive spatial density dependence persisted through four of five sequential week-long periods of data collection. Both patterns imply that invading galls are a significant resource for at least some native bird populations. Density dependence was strongest in southern UK bird populations that have had longest exposure to the invading gallwasps. We hypothesise that this pattern results from the time taken for native bird populations to learn how to exploit this novel resource.
Highlights
Understanding the interactions between alien species and native communities is central to understanding the ecological consequences of species range expansions
We examine predation by native U.K. insectivorous birds on a group of invading insect herbivores associated with an introduced oak – the Turkey oak, Quercus cerris
Statistical Methods We tested for spatial density dependence using generalised linear model (GLM) analysis of the proportion of predated galls as a function of gall densities per shoot (log(x+1) transformed) using R [35] with quasibinomial errors to correct for under-dispersion and a logit link function [36,37]
Summary
Understanding the interactions between alien species and native communities is central to understanding the ecological consequences of species range expansions. Biological invasions often disrupt existing networks of biotic interaction among species, with impacts on native species ranging from negative (reduced population densities, extinction) to positive Many aspects of this process have been investigated using natural systems comprising invading herbivorous insects and their parasitoid natural enemies [4,5,6,7,8,9,10]. While recruitment of parasitoid natural enemies to these invaders has been studied in considerable detail [9,10,13,14], much less is known of the development of trophic links with native birds
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